It is axiomatic that behavioral development is a function of both maturational processes and experience. For the most part, developmental psychology has focused on the experiential determinants of behavior. Recent advances in psychobiology, however, have led to a new appreciation of biological contributions to behavior, and research exploring this relationship is ongoing in many fields. In this context an obvious task for developmental psychology is to study the role of maturational processes in human behavioral development. My research focuses on maturation and, in particular, on the significance for human behaviroal development of variations in maturational rates. The specific studies outlined here use a neuropsychological model of brain-behavior relationships to examine different rates of physical and neurological maturation in relation to behavior during late childhood and adolescence. This period is unique in providing an opportunity to observe in the same individual both rapid development and the emergence of the mature adult state over a relatively short time span. In prior research, I investigated sex-related characteristics in order to determine biological contributions to the development of individual differences in patterns of mental abilities. It was known (1) that rate of physical maturation is sex-related (females mature earlier than males) and (2) that mental ability patterns are sex-related (females perform better at verbal than at spatial tasks; males show the opposite pattern.) I therefore investigated the relationship between rate of maturation and patterns of mental ability. The findings confirmed by hypothesis that irrespective of sex or chronological age, early maturers show the "female" pattern and late maturers the "male" pattern. In addition, they demonstrated that early and late maturers differ in the functional organization of the two cerebral hemispheres. This result suggests that the relationship between maturational rate and cognitive patterns has a basis in the functioning of the central nervous system. The studies outlined here provide for further investigation of these phenomena. The aims of the first study are (1) to observe the development of the relationships among rate of maturation, patterns of cognitive ability and functional lateralization by examining children prior to the onset of puberty and again as they reach puberty (permitting comparison of early and late maturers both before and after their status is apparent) and (2) to determine whether aspects of maturation and behavior not measured in my previous work are also interrelated. The aims of the second study are to determine (1) whether different ways of processing information are correlated with the different performance levels observed in early and late maturers and (2) whether the way the cerebral hemispheres interact in processing information is related to maturational rate and/or to cognitive performance. The broad goal of this research program is to use the study of maturational rates as a vehicle for exploring the nature of the biological mechanisms that underly human behaviroal development.